Incandescent lamps such as tungsten filament lamps or halogen lamps are widely used as sources of artificial light. In the early stage, incandescent lamps are used for simply providing a bright place. With diversified living attitudes, incandescent lamps having difference brightness are developed. For adjusting brightness of respective incandescent lamp, a brightness-adjustable circuit is used to drive the incandescent lamp and control the brightness of the incandescent lamp.
FIG. 1 is a schematic circuit diagram illustrating a brightness-adjustable circuit for a conventional incandescent lamp. As shown in FIG. 1, the brightness-adjustable circuit 1 includes a switch element 11 and a triggering circuit 12. The switch element 11 is for example a solid semiconductor component such as a silicon-controlled rectifier (SCR) or a TRIode for Alternating Current (TRAIC) component. Take a TRAIC component as the switch element 11 for example. The control terminal G is the gate of the switch element 11. The first terminal N1 and the control terminal G of the switch element 11 are coupled to the incandescent lamp 13 and the triggering circuit 12, respectively. The second terminal N2 of the switch element 11 can receive the electric energy from the input voltage Vin. The triggering circuit 12 can control the on phase or on duration of the switch element 11, thereby controlling the electricity to be transmitted to the incandescent lamp 13.
Please refer to FIG. 1 again. The triggering circuit 12 includes a resistor R, a variable resistor Rvar a capacitor C and a bidirectional diode thyristor D. The resistor R, the variable resistor Rvar and the capacitor C are connected in serried with each other to form a charging loop. Both ends of these serially-connected components are coupled to the second terminal N2 of the switch element 11 and the incandescent lamp 13, respectively. An end of the bidirectional diode thyristor D is coupled to the control terminal G of the switch element 11. The other end of the bidirectional diode thyristor D is coupled to the capacitor C. Through the charging loop which is defined by the resistor R, the variable resistor Rvar and the capacitor C, the input voltage Vin can charge the capacitor C. Until the capacitor C is charged to the turn-on voltage of the bidirectional diode thyristor D, the bidirectional diode thyristor D is conducted and thus a triggering signal is transmitted to the control terminal G of the switch element 11. In response to the triggering signal, the switch element 11 is conducted. That is, the on phase or on duration of the switch element 11 can be controlled by adjusting the resistance of the resistor R, thereby controlling the electricity to be transmitted to the incandescent lamp 13 and adjusting the brightness of the incandescent lamp 13.
In recent years, light emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFLs) that emit light with high brightness values and high illuminating efficiency have been developed. With the maturity of the LED or CCFL technology, LEDs and CCFLs will replace all conventional lighting devices in many aspects such as home-use lighting devices.
The conventional brightness-adjustable circuit 1, however, is only applicable to the incandescent lamp with the pure resistive property. If the conventional brightness-adjustable circuit 1 is applied to a cold cathode fluorescent lamp or a light emitting diode, the cold cathode fluorescent lamp or the light emitting diode fails to be normally operated and is possibly burnt out. In other words, the conventional brightness-adjustable circuit is not feasible to adjust brightness values of the cold cathode fluorescent lamp or the light emitting diode.
Therefore, there is a need of providing an improved brightness-adjustable illumination driving system so as to obviate the drawbacks encountered from the prior art.